TiNi shape memory alloy has been used in many application fields due to its excellent shape memory effect (SME) and superelasticity (SE). However, it is difficult and costly to machine TiNi alloy into complex shapes due to its low ductility. To address this problem, one approach is near-net shape processing by vacuum plasma spraying (VPS). In this study, the transformation behavior, mechanical properties and microstructure of TiNi alloy processed by VPS method are studied. The as-sprayed and homogenized TiNi alloy exhibited compositional variations in the sample, though both samples exhibited a single TiNi phase with low transformation temperatures, below 170 K Aging the homogenized sample at 773 K for 18 ks led to an increase in the transformation temperature, resulting in good transformation behavior. Specifically, DSC measurement revealed clear transformation peaks due to Martensite, austenite and R-phase transitions. Compression testing of a sample aged at 773 K for 18 ks exhibited a good SME below Mf and superelasticity (SE) above Af. The recoverable strain due to SME and SE were more than 2.4 % and 5.0 %, respectively. TEM studies confirmed that aTi3Ni4 precipitate was formed by aging at 773 K for 18 ks.
Ferromagnetic Shape Memory Alloy (FSMA) particulate composites are processed using Spark Plasma Sintering (SPS) with various weight fractions of NiTi (51 at% Ni) and Fe powders. Various processing conditions are experimented to obtain the optimum heating rate, holding time and holding temperature in order to maximize relative density, superelasticity and magnetic saturation of the composite. Mechanical strength is evaluated using compression tests.
Ferromagnetic Shape Memory Alloy (FSMA) particulate composites are processed using Spark Plasma Sintering (SPS) with various weight fractions of NiTi (51 at% Ni) and Fe powders. The magnetic properties of these composite specimens were experimentally evaluated using Vibration Sample Magnetometry (VSM). A model for calculating the effective magnetic properties has been presented in this work where Eshelby's inhomogeneous inclusion method considering Mori-Tanaka's mean field theory for larger concentrations of Fe has been used to predict the effective magnetic properties. The analytical results thus obtained are compared with experimental data resulting in a reasonably good agreement.
Dynamic deformation behavior of TiNi (superelastic grade) and TiNiCu alloy (shape memory grade) were examined using Split Hopkinson Pressure Bar. The stress-strain curves of the TiNi alloy exibits strain rate sensitivity. The flow stress in the plateau region increased with increasing of strain rate logarithmically, and the on-set stress for stress induced martensite also increased slightly. In contrast, the stress-strain curve of the TiNiCu alloy was found to be much less sensitive to strain rate. TEM observations revealed that the microstructure of the dynamically deformed TiNi is similar to that of the sample before dynamic deformation. In contrast, the dynamically deformed TiNiCu has a fine twinned structure than the sample deformed statically.
Analytical constitutive equation for the dynamic deformed TiNi alloy was proposed by addition of the terms concerning the strain rate effect and temperature change due to adiabatic deformation and latent heat of martensitic transformation, the revised constitutive equation was in a good agreement with the experimental results.